The present invention is directed to a system for delivering high energy laser by means by an optical waveguide, and in one particular application is concerned with laser angioplasty.
The use of laser energy to ablate atherosclerotic plaque that forms an obstruction in a blood vessel is presently being investigated as a viable alternative to coronary bypass surgery. This procedure, known as angioplasty, essentially involves insertion of a fiberoptic waveguide into the vessel, and conduction of laser energy through the waveguide to direct it at the plaque once the distal end of the waveguide is positioned adjacent the obstruction. In order to enable the physician to ascertain the location of the waveguide as it is being moved through the vessel, additional waveguides for providing a source of illuminating light and for conducting the image from inside the vessel back to the physician are fed together with the laser waveguide. Typically, the three waveguides are encapsulated within a catheter.
Most of the experimentation and testing that has been done in this area has utilized continuous wave laser energy, such as that produced by Argon Ion, Nd:YAG or Carbon Dioxide lasers. The light produced by this type of laser is at a relatively low energy level. Ablation of the obstruction is achieved with these types of lasers by heating the plaque with the laser energy over a period of time until the temperature is great enough to destroy it.
While the use of continuous wave laser energy has been found to be sufficient to ablate an obstruction, it is not without its drawbacks. Most significantly, the removal of the obstruction is accompanied by thermal injury to the vessel walls immediately adjacent the obstruction. In an effort to avoid such thermal injury, the use of a different, higher level form of laser energy having a wavelength in the ultra-violet range (40-400 nanometers) has been suggested. This energy, known as Excimer laser energy, can be provided, for example, by a laser medium such as argon-chloride having a wavelength of 193 nanometers, krypton-chloride (222 nm), krypton-fluoride (240 nm) or xenon-chloride (308 nm). The output energy from this type of laser appears in short bursts or pulses that can last for 10-85 nanoseconds and have a high peak energy, for example as much as 200 mJ. Although the destruction mechanism involving this form of energy is not completely understood, it has been observed that one pulse of the Excimer laser produces an incision which destroys the target tissue without accompanying thermal injury to the surrounding area. This result has been theorized to be due to either or both of two phenomena. The delivery of the short duration, high energy pulses may vaporize the material so rapidly that heat transfer to the non-irradiated adjacent tissue is minimal. Alternatively, or in addition, ultraviolet photons absorbed in the organic material might disrupt molecular bonds to remove tissue by photochemical rather than thermal mechanisms.
While the high peak energy provided by the Excimer laser has been shown to provide improved results with regard to the ablation of atherosclerotic plaque, this characteristic of the energy also presents a serious practical problem. Specifically, a laser pulse having sufficient energy density to destroy an obstructing tissue also tends to destroy an optical fiber. The high energy density pulses break the fiber tip at the input end, first at the glass/air interface. Continued application of the laser energy causes a deep crater to be formed inside the fiber. Thus, it is not possible to deliver high-power ultraviolet laser energy in vivo using a conventional system designed for continuous wave laser energy.
This problem associated with the delivery of high energy Excimer laser pulses is particularly exacerbated in the field of angioplasty because of the small optical fibers that must be used. A coronary artery typically has an internal diameter of two millimeters or less. Accordingly, the total external diameter of the angioplasty system must be below two millimeters. If this system is composed of three separate optical fibers arranged adjacent one another, it will be appreciated that each individual fiber must be quite small in cross-sectional area.
A critical parameter with regard to the destruction of an optical fiber is the density of the energy that is presented to the end of the fiber. In order to successfully deliver the laser energy, the energy density must be maintained below the destruction threshold of the fiber, which might be around 25-30 mJ/mm.sup.2. Thus, it will be appreciated that fibers having a small cross-sectional area, such as those used in angioplasty, can conduct only a limited amount of energy if the density level is maintained below the threshold value. This limited amount of energy may not be sufficient to ablate the obstructing tissue or plaque.